1. Field of the Invention
This invention relates to a digital audio signal reproducing apparatus in which compressed data of digital audio signals recorded of a recording medium is read out from the recording medium and written in a memory so as to be read out from the memory at a constant data rate.
2. Description of the Related Art
The present Assignee has proposed a technique of bit compressing input digital audio signals and recording the data of a predetermined data volume as a recording unit in a burst-like manner.
With this technique, a magneto-optical disc is used as a recording medium for recording and/or reproducing adaptive differential (AD) PCM audio data as prescribed in e.g. CD-I (CD-Interactive) or CD-ROM XA audio data formats, or digital audio data encoded with bit compression in accordance with other formats than those given above. The bit-compressed digital audio data are recorded sector-sequentially and intermittently, that is, in a burst-like manner, with addition of linking sectors before and after each recording unit of a predetermined data volume, such as 32 sectors, to account for interleaving with the data of neighboring sectors.
It is now assumed that data of a so-called standard compact-disc (CD) format (CD-DA format) or so-called straight PCM audio data produced upon simple linear quantization of analog audio signals, are recorded and/or reproduced with approximately 1/4 bit compression. The play time of a disc on which data are recorded with approximately 1/4 bit compression is approximately 4 times the time necessary for recording the straight PCM data prior to bit compression, such as the above-mentioned CD-DA format data. Since the recording/play time approximately equal to that in the case of a standard 12 cm CD may be achieved with a disc of a smaller size, the apparatus may be reduced in size. On the other hand, by setting the instantaneous bit rate for recording/reproduction so as to be equal to that of the above-mentioned standard CD-DA format, the time necessary for recording/reproduction may be reduced to about 1/4 of the time, so that the remaining 3/4 of the time may be allocated to so-called retry or the like operations. These operations include a verification operation, which verifies if recording has been made regularly, or a rewriting operation in case of a recording failure, as far as data recording is concerned, and a re-reading operation in case of a high data read error rate, as far as data reproduction is concerned. This system may be advantageously employed for a portable small-sized apparatus because recording/reproduction may be achieved more reliably even under adverse conditions of defocusing or detracking due to vibrations of the mechanical components caused by disturbances.
For recording/reproduction of bit compressed digital audio data to a volume 1/4 of the original data, a buffer memory for recording/reproducing the compressed data is required. The compressed data is written continuously at a constant rate during recording and read out intermittently (in a burst fashion) at a rate about four times the write rate. The data volume read out intermittently each time is e,g. 32 sectors. As described above, several linking sectors are added before and after these 32 sectors and the resulting recording unit is recorded on the disc in spatial continuation to the preceding recording unit. During reproduction, the data volume of the above-mentioned recording unit, consisting of the 32 sectors and several linking sectors before and after the 32 sectors, is intermittently reproduced from the disc at a read-out rate about four times the write rate, and the linking sectors before and after the 32 sectors are removed before the data are written in the buffer memory. It is from this memory that the compressed data is read out continuously at the above-mentioned constant rate.
The total storage capacity (M.sub.T) of the above-mentioned recording and/or reproducing buffer memory, is equal to the sum of the data volume of the compressed data written in and not read from the buffer memory (volume of non-read-out data) and the remaining storage capacity of the buffer memory, that is the volume of the data that can be written in the buffer memory without destructing the non-read-out data (volume of writable data). The following is an explanation of the controlling of data writing and read-out in or from the memory.
This controlling is made such that, during recording, the compressed data is written at a constant data rate into the buffer memory and, when the volume of the non-read-out data exceeds a predetermined volume M.sub.K, data is read out in a burst-like manner for each recording unit consisting of, for example, 32 sectors plus several sectors, at a readout rate faster than the data write rate, so as to be recorded on a recording medium, such as a disc. Data not recorded as yet on the recording medium represents the non-read-out data on the memory. If the recording on the recording medium is interrupted by disturbances, such as vibrations, or if recording has not been made normally, the volume of non-read-out data in the memory is not decreased, such that, if the volume of the non-read-out data is close to the total storage capacity M.sub.T of the memory, the non-read-out data may be destroyed by the compressed data written at a constant rate. In view of this consideration, recording on the recording medium is carried out under a condition in which the writable capacity in the buffer memory is below (M.sub.T - M.sub.K), that is in which there is a certain allowance in the storage capacity of the writable area, so that destruction of the non-read-out data may be inhibited even in cases wherein recording on the recording medium is interrupted or is not carried out in a regular manner. It suffices to set (M.sub.T -M.sub.K) so that a time interval which would be required for reversion to the state of recording on the recording medium from the state of recording interruption and for re-recording is presupposed and the operation of writing the compressed data on the memory may be continued during this presupposed time interval.
The controlling during reproduction is so made that, while the compressed data, read out in a burst-like manner from the recording medium, is written in the memory so as to be read at a constant data rate, the non-read-out data is read out from the recording medium in a burst-like manner so as to be written in the memory when the data volume of the non-read-out data becomes less than the predetermined data volume M.sub.L. Accordingly, even if data readout from the recording medium cannot be made in a regular manner due to disturbances or the like, the remaining data volume M.sub.L may be continuously read to prevent data reproduction from being interrupted due to interruption of the read-out operation at the constant rate. It suffices to set the non-read-out data volume M.sub.L so that the compressed data may be continuously read at the above-mentioned constant rate from the memory at the above-mentioned constant data rate for a time required for re-reading the recording data from the recording medium.
Meanwhile, in a so-called CD (compact disc) player, there is frequently provided a pause function for transiently discontinuing the playback operation, an error selecting and reproducing function for selecting an error and reproducing the selected error, a one/all repeat function for repeatedly reproducing one or all of the errors of the disc, or an A/B repeat function of repeatedly reproducing from an arbitrary point A to another arbitrary point B, for thereby making the best use of accessibility proper to the disc. By taking advantage of these functions, not only is the operating labor dispensed with, but a more versatile domain of audio entertainment may be achieved.
If these functions are to be annexed to the recording/reproducing apparatus or reproduce-only apparatus for compressed data as described above, a time lag is produced between the audio signals being reproduced and audio signals being read from the disc, due to the use of the buffer memory for reproduction, thus presenting response problems. Besides, a simplified arrangement for annexation of these functions is preferred.